As communication technologies are improved and various communication services are developed, it is requested for the communication systems to allocate additional frequency bands.
However, there is no room for the next generation communication service to allocate the available low frequency bands, because almost frequency bands are already licensed and allocated for the existing broadcast and communication services.
In order to overcome such problem of the lack of available low frequencies, a cognitive radio (hereinafter, referred to as ‘CR’) has been proposed. The CR is capable of detecting an empty frequency within a licensed frequency band and using the temporarily empty frequency adaptively.
The spectrum sensing of the CR is executed to coexist a primary user (licensed user) with priority and a secondary user (unlicensed user) without priority upon the spectral use. The secondary user may determine whether the primary user has occupied any frequency band through the sensing of frequency spectrum so as to prevent a collision occurrence upon using the same frequency with the primary user.
FIG. 1 shows a transmission of data frames using the spectrum sensing of the CR technology.
The spectrum sensing may repeat sensing operation and transmission operation with respect to a frequency occupation of the primary user as shown in FIG. 1. If data transmission is needed to the secondary user, spectrum sensing is executed for the frequency band F to check whether there is a transmission opportunity to the secondary user. After executing the spectrum sensing for the frequency band F during a sensing time nD, the secondary user starts transmission if there is not detected any packet transmission of the primary user.
However, if the transmitted data of the secondary user are all sent at once, there is no way to check whether the primary user occupies the channel during the data transmission of the secondary user. Accordingly, as shown in FIG. 1, a data packet is split into several packets 101, 103 and 105 and sensing is executed in each section between the split data packets. If a packet which is being transmitted from or to the primary user is not detected, the secondary user temporarily occupies the channel again so as to transmit data.
Therefore, the procedure of transmitting data by the second user has a repeating operation of ‘sensing (nD)-transmission (t1)-sensing (nD)-transmission (t2)-sensing (nD)-transmission (t3)’.
According to the existing approach, the repeating operation of sensing and transmitting has a structure in which a length of sensing slot and transmission slot are always uniformly repeated.
However, the periodical sensing approach has a drawback of failing to reflect the increased probability that the primary user will come back to the channel as time elapses after a secondary user starts data transmission. Consequently, when the secondary user transmits data for a long period of time, the collision probability between the data transmission of secondary user and the primary user may problematically increase.
Referring to FIG. 1, during a transmission time interval t3 of the third frame 105 of the secondary user, the primary user occupies a channel to transmit a packet 200, thereby causing the collision 109 between the transmission packet 200 of the primary user and the transmission packet 105 of the secondary user.